Molecular chaperons of the Hsp70 family bind unfolded polypeptide substrates to stabilize or alter their conformation and hydrolyze ATP to facilitate polypeptide release. The ATP hydrolysis of Hsp70 is regulated by another protein chaperon family Hsp40. It has been reported that Hsp40 my bind polypeptides with secondary structure, interact directly with Hsp70 and stimulate the ATP hydrolysis of Hsp70. SIS1 is a member of Hsp40 protein family in Yeast Saccharomyces cerevisiae and essential for cell viability. We have crystallized the protein SIS1 recently, the crystals diffract to 2.7 E. Selenomethionyl SIS1 has been expressed and purified from E. coli strain B834. Similar crystals are obtained using the same conditions. This proposal requests beamtime to use the MAD method to solve the 3-D crystal structure of protein Hsp40.